This invention is related to a method and an equipment for the measuring and determining in an electric way by means of a common electrode at least two components, for instance pH and pO.sub.2, which are parts of the electrode potential of the electrode. In particular, the invention concerns measuring of pH and pO.sub.2 with one and the same antimony electrode.
The measuring of bloodgases constitutes one of the cornerstones within the medical diagnostics. The oxygen value pO.sub.2 describes how well the blood is saturated by oxygen in the lungs and how good the saturation by oxygen is in relation to the oxygen needs of the tissues, and the blood becomes acidified, that is the pH-value is reduced when the requirements are greater than the available oxygen. Another central parameter in the analysis of bloodgases is the carbon dioxide level, pCO.sub.2, which is a measurement of how well the sugar decomposing products are ventilated from the lung.
Since the middle of the 19the century, it has been a goal in the art to find good systems for the measuring of pH, PO.sub.2 and pCO.sub.2 in blood and tissue.
The standard method for measuring pH has since long time being with glass based electrodes. The drawback of these have been that they are brittle and suffer from drift, which contributes to the impossibility of using them for continuous measuring in blood or tissue or for instance, control of patients.
The standard method for measuring oxygen levels in medical appliances has been by means of a Clark electrode. This has a complex structure including the enclosing of the electrode in a number of membranes and corresponding electrolytic solutions to provide a stable measuring system. The complexity of the construction and the drift of the measuring device has, however resulted in that it is not possible to use, for a clark electrode continuous measurements for supervising or control proposes. The tendency of thrombosis continuous around the electrode has further contributed to the difficulties.
Also, for the measuring of carbon dioxide continuously in the bloodstream or tissue of patients a good method and equipment is missing today. The traditional equipment for this is based on a pH-electrode which is placed inside a membrane that is previous to carbon dioxide. The solution inside the membrane is a buffer that, through the addition of the carbon dioxide from the measured environment, will become more acid when the carbon dioxide is dissolved in the fluid to make carbonic acid. The acidification inside the membrane is then proportional to the carbon dioxide levels outside the membrane, and can be measured by means of a conventional pH-electrode.
From the Swedish patent document 409 372 a metal electrode for the measuring of pH, etc. in liquids is previously known. This electrode consist of a sensor of metal enclosed in a holder; the metal is monocrystaline with only one crystal plane exposed to the testing liquid. With such an electrode made of antimony, pH measuring with high resolution (0.02 pH-units) is made possible. This electrode type has the same good characteristic for pH measuring as glass-based pH electrodes and have several practical advantages in comparison to the glass electrode since the electrode type is less vulnerable, easy to sterilise and not harmful to biological tissue.
A remaining problem with antimony-based electrodes is however, the oxygen dependency. This has been investigated in a number of studies, where among other things, it has been shown that the oxygen-dependent part of the electrode voltage is stable, thereby enabling simultaneous oxygen measuring, provided that the pH component can be identified and subtracted. In the described case (Sjoberg, P. Skeletal muscle surface pH and PO.sub.2. Linkoping University Medical Dissertations No 325, 1990. Linkoping), where the electrode has been used for tissue measuring, this has been carried out by for a short time cutting off the flow of blood to the tissue. This results in the oxygen pressure for a short time becoming 0 and it is thereby possible to identify pH part of the signal. By then subtracting the pH part of the signal, the oxygen levels before the cutoff can be calculated. The drawback with this method is that the cutoff may influence the cutogg environment of the measuring. And, some tissues cannot endure the cutoff method, for instance the central nervous system. It should therefore be of a great value to be able to separate the two components of the electrode voltage in an electrical manner.